craters themselves have filled with water following the surface bursts in the Pacific, and thus hold that a true land burst would give higher crater intensities; others contend that the deposition mechanisms change with yield, so that the increased scouring effect and higher cloud rise of large yield weapons would tend to keep the crater and lip dose rates at H+l hour more or less constant with yield. A firm resolution of this uncertainty is not likely until a test detonation of a medium or large yield weapon is held on a truly representative dry land surface. The time of arrival of fall-out particles of various sizes from atomic clouds can be estimated using Figure 3, which gives cloud heights, in conjunction with Figure 5, which gives the times for par- ticles of various representative surface burst weapons with yields between 1 KT and 500 KT. The 1000 micron size is probably representative of early fall-out arrival, mainly in the area near the burst point; while the 75 micron size is representative of the fall-out likely to occur in the downwind extremes of the elliptical patterns. TABLE 3 Total Cloud Bot- Yield tom (ft) 1k 5 kT 10 KT 100 KT 500 KT 4,000 15,000 20,500 39,000 51,000 Cloud Bottom of Cloud Top_of Cloud _Top(ft) 1000» 175» 75 1000 n 175 9,000 24,000 31,000 54,000 70,000 15min 30min 15 "75 20 "90 45 "150 55 "160 90 min "180 " "300 " "480 " "650 " 15 min 30 ho 50 60 " “ " ” 75 4p 60 min 150 min 90 120 180 210 " " " * 300 hao 700 &50 " " ” " In evaluating the data presented and the material discussed on the problem of the areas involved in close-in fall-out, it must be remembered that most of the information has been gathered from controlled field test operations. Winds at all altitudes were known to the best of the ability of the meteorologists and shot days were selected so that the best conditions existed to minimize the hazard from close-in fall-out. Uncertainties in the information presented are due primarily to the question as to whether the burst conditions wnder which the test hig